Unless I've misunderstood something about that. It happens.
We do the redundant Active Directory thing. It didn't help during the DR exercise when the AD guy did something foolish (don't remember what) and the AD / DNS host went down for a few hours.
Single host because the DR was limited in scope.
I was fine with my Solaris hosts - had local root access via serial and SSH. I was simply locked out of my Windows hosts, and could not reconfigure those services to work without AD.
You just maintain a local/serial-only root account for that eventuality.
[Edit] And make sure internet-facing production services don't rely on administrative LDAP.